Abstract
Potato is an important cultivated crop species and since it is moderately salt sensitive there is a need to develop more salt tolerant cultivars. A high activity of Na(+) transport across the tonoplast in exchange for H(+) is essential to reduce Na(+) toxicity. The proton motive force (PMF) generated by the V-H(+)-ATPase and the V-H(+)-PPase energizes the Na(+)(K(+))/H(+) antiport. We compared the activity, gene expression, and protein levels of the vacuolar proton pumps and the Na(+)/H(+) antiporters in two potato cultivars (Solanum tuberosum) contrasting in their salt tolerance (cv. Desiree; tolerant and Mozart; sensitive) grown at 0 and 60 mM NaCl. Tonoplast-enriched vesicles were used to study the pump activity and protein levels of the V-H(+)-ATPase and the V-H(+)-PPase and the activity of the Na(+)/H(+) antiporter. Although salt stress reduced the V-H(+)-ATPase and the V-H(+)-PPase activity in both cultivars, the decline in H(+) pump activity was more severe in the salt-sensitive cultivar Mozart. After salt treatment, protein amounts of the vacuolar H(+) pumps decreased in Mozart but remained unchanged in the cultivar Desiree. Decreased protein amounts of the V-H(+)-PPase found in Mozart may explain the reduced V-H(+)-PPase activity found for Mozart after salt stress. Under non-stress conditions, protein amounts of V-H(+)-PPase were equal in both cultivars while the V-H(+)-PPase activity was already twice as high and remained higher after salt treatment in the cultivar Desiree as compared to Mozart. This cultivar-dependent V-H(+)-PPase activity may explain the higher salt tolerance of Desiree. Moreover, combined with reduced vacuolar H(+) pump activity, Mozart showed a lower Na(+)/H(+) exchange activity and the K(m) for Na(+) is at least twofold lower in tonoplast vesicles from Desiree, what suggests that NHXs from Desiree have a higher affinity for Na(+) as compared to Mozart. From these results, we conclude that the higher capacity in combination with the higher affinity for Na(+) uptake can be an important factor to explain the differences in salt tolerance of these two potato cultivars.